To Format a Hard Drive for Maximum Windows 7 and Mac OSX Compatibility set the Cluster Size Equal to or Less Than 1024 Bytes
I wish I had read this helpful post by Kris Haamer before I made the same mistake while formatting a mac/pc compatible raid using two 4 tb drives.
“For anyone trying to use an external HD to swap files between an Apple and a Microsoft computer. In theory, MacOS and Windows 7 are both fully compatible with the exFAT file system (MacOS since version 10.6.5. and Windows since XP) but in reality when formatting your HD into exFAT you need to make sure to set the cluster size equal to or less than 1024 bytes. If you set the value higher you’ll get a “unable to mount” error on a Mac”
– Kris Haamer